The ten-year (1993-2002) BLSA Study of Prostate Aging and Disease has both retrospective and prospective arms involving repeated assessments of anatomical, physiological, hormonal, and behavioral aspects of age-associated changes in prostate size. A major goal of the study is to identify antecedents of prostate cancer and benign prostatic hyperplasia (BPH). Current research addresses the question of when serial measures of PSA are not necessary, an interesting question because of the current controversy in medical practice about the clinical value of repeated measures of PSA. Men in three cohorts aged 60, 65 or 70 years with initial PSA levels equal to or lower than 1.0 ng/ml were compared with those with initial levels greater than 1.0; changes in PSA levels over periods ranging from 5 to 15 years in both groups were followed until diagnosis of cancer (cases) or age 75 (noncases). The probability of converting to a level of 4.0 ng/ml--the level at which PSA monitoring is considered clinically indicated--was compared in the two groups. Conversions to PSA levels of 4.0 or greater were lower among men with initially low PSA values. For example, 1 of 82 65-year-old men with PSA levels equal to or below 1.0 ng/ml developed prostate cancer by age 75 while 20 of 100 with initial values greater than 1.0 ng/ml. Corresponding figures for 60-year old men were 4/112 vs. 17/74; for 70 year-old men, the corresponding figures were 1/46 vs. 14/82. Thus, if 65-year olds were no longer PSA tested, then 45% of them would not require further PSA testing and 95% of the cancer cases would still be detected. The results suggest that a testing protocol that would stop PSA monitoring in older men with initially low PSA levels may be considered in clinical practice.